This invention relates to pacemaker systems and the use thereof, and in particular pacemaker systems which are automatically rate adaptable.
The use and employment of rate adaptive or physiological pacemakers is now gaining an increasing acceptance, although it has been slow coming. The principle of using atrial activity to trigger ventricular pacing has been implemented in implantable devices for about two decades, but even to date such pacemakers are very much in the minority. However, the reasons for the historical limited acceptance of atrial synchronous pacemakers, and other types of dual chamber pacemakers, are being overcome by the industry. The problems with reliable sensing of atrial activity, bulky short-lived generators and ventricular competitive pacing are being solved in varying degrees.
A clear understanding of the factors responsible for the adjustment of cardiac output in relation to metabolic needs, and interest in sensing arrangements other than the atrium to provide physiological information, has contributed to evoking technical development in this area. In short, the object is to define a pacing system able to confer physiological benefit under a wide variety of conditions and cardiac rhythms. The ability to adjust cardiac output in relation to metabolic needs is a fundamental property of the intact cardiovascular system, and increase of heart rate alone can increment cardiac output by 300 percent in normal adults; combined with stroke volumne increases of around 50 percent this produces the normal 4 to 5 - fold increase of cardiac output found in exercise. In patients with complete heart block the loss of rate control thus has a profound effect on exercise haemodynamics. Thus, loss of rate control applies not only to patients with atrioventricular disociation, but also to subjects with an abnormal sinus response to exercise, even though atrial synchrony may be maintained.
Two main approaches currently exist in the design of physiologically adaptive pacemakers. The first is the dual chamber pacemaker, e.g. use of atrial synchronous ventricular pacing. If the atrium is normally responsive to metabolic demands, this pacing system represents the ideal pacing modality. Such a pacing system has the advantage that it not only provides rate control mediated by the sinus node, but also allows physiological A-V synchrony to be maintained. The haemodynamic benefit of dual chamber devices (including VDD and DDD) has been established both in the long and short term, and the use of such in patients with predominated or exercise-induced high grade A-V block and normal sinus function is clearly mandated.
There remain, however, some technical problems related to the use of atrial synchronous pacemakers. The incidence of pacemaker mediated tachycardia is a problem that the industry has been working on vigorously. One of the predominant problems that must be solved in this area is that of achieving the ability to sense the difference between physiological and pathological atrial tachycardias.
At the present time it is estimated that over half the patients receiving implanted cardiac pacemakers have abnormal atrial functions. Primary sino-atrial disease is present in 48% of patients presenting for pacing in the U.S., and many of the remaining patients will have associated sino-atrial disease, or will develop atrial arhythmias during the course of their life. Thus, perhaps less than 1/3 of prospective patients with symptomatic bradycardia have a normal sinus node which can be used as a faithful determinant of metabolic demand.
The above considerations have stimulated interest in a second approach to the design of physiologically adaptive pacemakers. Such second approach system produces changes in the ventricular rate by using other means of sensing metabolic demand or exercise itself, independent of atrial activity. Ventricular pacing systems now in development have been based on varying principles. The QT sensitive pacemaker responds to an indication of metabolic demand and therefore is sensitive to the effects of emotion and those cardioactive drugs which alter the catecholamine sensitivity of the heart. Reference is made to U.S. Pat. Nos. 4,228,803, Rickards, and 4,305,396, Wittkampf et al, which describe embodiments of a evoked QT sensing type pacemaker. A pacemaker using the basic evoked QT sensing arrangement has been developed by the assignee of this application, and is hereinafter referred to as operating in the T.sub.x mode. The advantages of this system include its inability to induce pacemaker mediated tachycardias. Of course, it does not provide suitably timed atrial contraction.
In addition to the T.sub.x mode rate adaptive pacer, other systems are under development which use body sensors for monitoring different body parameters, the pacemaker having means for controlling and adapting rate as a function of the sensed body parameter. Such systems include sensors for pH control, control by central venus oxygen content, temperature control, and resperation rate control. Reference is made to the paper titled "Rate Responsive Pacing", Rickards et al, Vol. 1, No. 1, 1983, Clin. Prog. Pacing and Electrophysiol.